Known in the art is an internal combustion engine arranging a particulate filter carrying a catalyst in an engine exhaust passage, arranging a selective reduction catalyst in the exhaust passage downstream of the particulate filter, and feeding urea into the exhaust passage upstream of the particulate filter (for example, see Japanese Patent Publication (A) No. 2004-511691). In this internal combustion engine, when the catalyst carried on the particulate filter is activated, the unburned HC and CO contained in the exhaust gas are removed by oxidation at the particulate filter and the particulate contained in the exhaust gas is burned off on the particulate filter. Furthermore, in this internal combustion engine, the NOx contained in the exhaust gas is reduced on the selective reduction catalyst by the urea fed into the exhaust passage.
However, if urea is fed into the engine exhaust passage in this way, the intermediate product derived from this urea will react the unburned HC contained in the exhaust gas and produce harmful hydrogen cyanide HCN. In this case, when a large amount of unburned HC is exhausted from the engine such as particularly at the time of engine cold start, and at the instant when the unburned HC in the exhaust gas increases during vehicle operation, a large amount of hydrogen cyanide will be produced. As a result, the problem arises of a large amount of hydrogen cyanide HCN exceeding the allowable concentration being exhausted into the atmosphere.